This invention relates to hydrogen gas generating compositions and more specifically to compositions for generating hydrogen gas at an adjustable rate and at relatively low temperatures.
Simple means for generating relatively small amounts of hydrogen gas are desirable for many applications. Examples are inflation of lighter-than-air devices such as balloons and buoyant markers. Obviously, hydrogen gas thus generated is also suitable for use as a reactant in chemical reactions. Parameters, which are critical for those and other applications, include the ratios of the volume of hydrogen gas generated to the weight and volume of the reactants, both ratios preferably being as high as possible. Other parameters are the hydrogen gas evolution rate, the hydrogen gas temperature, the expendability of the gas generating systems after use, or a combination of the above. As is readily apparent, conventional methods such as using a pressurized hydrogen gas cylinder or generating hydrogen from the reaction of metals or metal hydrides with acids or water do not meet the above criteria.
U.S. Pat. No. 3,734,863 to Beckert et al. issued May 22, 1973, which is incorporated herein by reference, describes a method for generating hydrogen gas on a small scale under conditions which satisfy most of the above parameters. The method is based on the reaction of ammonium or hydrazonium salts with suitable metal hydrides as expressed by the following general equations: EQU (m/n) (NH.sub.4).sub.n X + Y(ZH.sub.4).sub.m .fwdarw. YX.sub.m/n + m ZN + 4m H.sub.2 EQU (m/n) (N.sub.2 H.sub.6).sub.n X.sub.2 + 2y(ZH.sub.4).sub.m .fwdarw. 2YX.sub.m/n + 2m ZN + 7m H.sub.2
where X is an acid group such as an inorganic acid group like halogen (Cl, Br, F), sulfate (SO.sub.4), and the like, n is the valency of the acid group, Y is a mono-or divalent metal capable of forming complex hydrides such as alkali and alkaline earth metals like Li, Na, K, Mg, Ba, Ca, etc., m is the valency of said metal and Z is a trivalent metal capable of forming complex hydrides such as B, Al, and the like.
Some of these reactions are highly exothermic as can be seen from the data in Table I, resulting in high gas temperatures.
TABLE I ______________________________________ CALCULATED HEATS OF REACTION H(kcal) H/mole H.sub.2 (kcal) 1. NH.sub.4 F + LiBH -20.36 -5.1 2. NH.sub.4 F + LiAlH.sub.4 -66.2 -16.6 3. NH.sub.4 Cl + LiAlH.sub.4 -53.82 -13.5 4. NH.sub.4 CL + NaAlH.sub.4 -53.5 -13.4 5. NH.sub.4 Br + LiAlH.sub.4 -50.61 -12.7 6. NH.sub.4 I + NaAlH.sub.4 -51.24 -12.8 7. N.sub.2 H.sub.6 Cl.sub.2 + 2 LiAlH.sub.4 -168.4 -24.1 8. N.sub.2 H.sub.6 SO.sub.4 + 2 NaAlH.sub.4 -160.7 -23.0 9. NH.sub.4 F.HF + 2 LiAlH.sub.4 -107.1 -16.5 ______________________________________
Experimentally it is found that mixtures consisting of NH.sub.4 Cl/NaAlH.sub.4 or NH.sub.4 F/LiAlH.sub.4 in stoichiometric proportions and containing varying amounts of binder reliably produce hydrogen gas in a short time with yields between 0.7 and 1.0 l/g; 100 l of hydrogen can be produced in less than 15 seconds, using these mixtures. However, the recorded gas temperatures are as high as 500.degree.C, or higher depending on the nature and amount of the material reacted and on the mode of firing. The high gas temperature prevents this hydrogen gas generating system from being used as a direct hydrogen source for balloon inflation, as outlined below.
In order to inflate a balloon with hydrogen the gas has to be relatively cool for two reasons:
1. The dependence of the mechanical properties of the balloon material on temperature sets an upper temperature limit.
Suitable balloon materials include natural or synthetic resins such as rubber, or polyethylene terephthalate (trademarked Mylar)
2. When a balloon is inflated to volume V.sub.o with hot hydrogen of temperature T.sub.o (.degree.K) without over-pressurization, and the gas, after removal of the gas source, subsequently cools to temperature T.sub.1 (.degree.K), a volume decrease .DELTA.V takes place which is, according to the ideal gas law, ##EQU1## resulting in a lift decrease of ##EQU2##
The variety of applications for a composition which generates hydrogen gas requires both generation of hydrogen at a relatively low temperature and a means for either increasing or decreasing the rate of hydrogen evolution. An increasing rate of hydrogen evolution is desirable for inflation purposes and some chemical reactions. A decreasing rate of hydrogen evolution is useful for either gas laser applications, other chemicals reactions, or fuel cells. A composition, which has an adjustable gas evolution rate is desirable because of the simplicity of varying a standard composition compared to the difficulty of maintaining a variety of compositions for each individual use.